Introduction
Tissue engineering aims to address the challenges of tissue degradation and regeneration through the integration of engineering and health sciences. In this field, the design and development of suitable scaffolds play a crucial role in promoting tissue growth and regeneration. The current study focuses on the synthesis of a Sr-doped multilayer ceramic-glass-ceramic scaffold that combines mechanical strength with bioactivity. Sr2+ ions, commonly found in bone, have been shown to play a vital role in regulating osteoblast activity and restricting osteoclast differentiation, making them an attractive option for incorporating into ceramic and glass materials. The goal of this research is to create a scaffold that not only offers mechanical stability, but also supports tissue growth and regeneration.
 
Experimental
The multilayer ceramic-glass-ceramic scaffolds were synthesized using a combination of sol-gel and polymer replication methods. A polyurethane sponge served as the template for the 3D scaffold structure. The sponge was impregnated with a dicalcium silicate ceramic and sintered at 1050°C for 8 hours to form the scaffold's core.
To enhance the mechanical properties of the scaffold, a Ca2P6O17 glass with 0.007 mol% Li2O was coated on the sintered core and sintered again at 1050°C for 8 hours. To modulate the scaffold's bioactivity, a dicalcium silicate ceramic doped with 1-5% strontium ions was applied as the final coating layer, followed by another sintering process at 1050°C for 8 hours.
The scaffolds were characterized using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and porosimetry techniques (BET-BJH). In vitro bioactivity was evaluated by immersing the samples in a simulated body fluid (SBF) for different time periods according to ISO 23317-2014.
 
Results and Discussion
The results of the XRD characterization show that the ceramic-glass (core) scaffold is mainly constituted by Ca2P2O7, CaSiO3, and LiCa(PO4) phases. The presence of pyrophosphate was evidenced through FTIR and SEM. The macroporosity of the ceramic-glass-ceramic was on average of ~90%, reaching microporous and mesoporous in smaller quantities. The ceramic-glass-ceramic scaffold shows a compression strength of 0.9-2.0 MPa. Finally, the bioactivity of the core coated with 1-5% Sr-doped ceramic layers was evaluated by its ability to precipitate hydroxyapatite on the surface when submerged in SBF. The ceramic scaffolds doped with 1% showed bioactivity after only 1 day of immersion in SBF, maintaining until the 21 days of assay.
 
Conclusions
The development of 3D ceramic-glass-ceramic scaffolds with optimal porosity and improved mechanical properties was achieved in this study. The scaffolds exhibited pyrophosphate and wollastonite as the main phases, both important regulators of the bone mineralization process. Additionally, the scaffolds doped with strontium at a concentration of 1-5% showed remarkable bioactivity after only one day of immersion in SBF. These scaffolds present a promising solution for bone tissue replacement and have the potential for various medical applications. However, further biological testing is required to confirm their full potential for use in medical practices.
 
Acknowledgements This work is part of the project PID2020-116693RB-C21 funded by MCIN/AEI/10.13039/501100011033 Spain. Grant CIAICO/2021/157 funded by Generalitat Valenciana Spain.